1. Field of the Invention
The present invention relates to communication systems, and more specifically to duobinary transmitters including optical duobinary transmitters designed for fiber-optic communication systems. Further, the present invention relates to an apparatus and method for optimizing electrical and optical duobinary signals against corresponding system impairments.
2. Description of the Related Art
The optical duobinary modulation format has been attracting increasing attention in recent years due to its many advantages for achieving both high-speed and high spectral efficiency in dense wavelength division multiplexing (DWDM) fiber-optic communication systems. Duobinary modulation produces a relatively narrow signal spectrum, relaxing the bandwidth requirements on components such as optical modulators and DWDM filters, while enabling a narrower channel spacing [Gill et. al., “42.7-Gb/s Cost-Effective Duobinary Optical Transmitter Using a Commercial 10-Gb/s Mach-Zehnder Modulator With Optical Filtering,” IEEE Phot. Tech. Lett., Vol. 17, No. 5, p. 917, April 2005].
For example, as compared to ON-OFF Keying (OOK) modulation, duobinary modulation requires approximately half the bandwidth, enabling an efficient transmission of twice the information per channel [Bigo et. al., “Multiterabit/s DWDM Terrestrial Transmission With Bandwidth-Limiting Optical Filtering,” IEEE J. Sel. Top. Quant. Elect., Vol. 10, No. 2, p. 329, March/April 2004.]. FIG. 1A compares the measured optical signal spectra for several optical modulation formats at 10 Gb/s bit rate. Note the relatively narrow duobinary spectrum (generated using a low-pass filter (LPF) implementation of duobinary generating filter) compared with OOK and differential phase shift keying (DPSK). The narrow duobinary spectrum enables duobinary systems to achieve 80% spectral efficiency DWDM transmission, without suffering significant penalties due to crosstalk or distortion from narrow optical filtering, as would be the case for other formats such as OOK or DPSK [Bosco et. al., “Modulation Formats Suitable for Ultrahigh Spectral Efficient WDM Systems,” IEEE J. Sel. Top. Quant. Elect., Vol. 10, No. 2, p. 321, March/April 2004.].
High-speed optical transmitters typically employ a Mach-Zehnder Modulator (MZM) for modulating an optical carrier wave with the electronic data. For both DPSK and Duobinary formats, the MZM is biased at a null, and driven between transmission maxima to enable phase modulation in DPSK, and a combination of phase/amplitude modulation in Duobinary [Winzer et. al., “Advanced Modulation Formats for High-Capacity Optical Transport Networks,” J. Lightwave Tech., Vol. 24, No. 12, p. 4711, Dec. 2006]. However, the Duobinary MZM requires approximately half the bandwidth compared with a DPSK MZM, a major advantage for high-speed transmission systems. Since a DPSK MZM requires greater bandwidth, it is more difficult and generally more expensive to manufacture compared with a Duobinary MZM. Moreover, as the demand for higher bit rates increases, the MZM bandwidth must increase accordingly. At some futuristic bit rate (e.g. 160 Gb/s), a DPSK MZM may become impractical, while a duobinary MZM will still be practical, as it requires much less bandwidth.
A conventional optical duobinary transmitter is shown in FIG. 1B. The transmitter shown is based on a dual-drive MZM 100B, such as for example a Z-cut Lithium Niobate MZM driven in a push-pull arrangement. In the dual-drive MZM, the two modulator arms are driven by the same voltage amplitude but in opposite directions in a push-pull operation. It is also possible to use a single-drive MZM, such as an X-cut Lithium Niobate MZM, in which case only a single drive voltage signal is required to drive both arms of the MZM simultaneously. The dual-drive MZM shown in FIG. 1B has the advantage of a lower drive voltage requirement compared with the corresponding single-drive MZM. The optical transmitter includes a binary data source (BDS) 110, which in practice may be a high-speed electronic multiplexer (Mux) used to multiplex slower speed tributary data signals up to the system bit rate. The optical duobinary transmitter further includes a duobinary precoder 120, differential driver amplifier (DA) 150, and two identical electronic duobinary generating filters (Duob Filters) 130 and 140. A laser source (LS) 160 is included along with a duobinary MZM 170. Note the DA 150 has two complementary outputs, amplified binary data signals B and B.
In operation, BDS 110 generates electronic binary DATA which is input to the precoder 120. The precoder outputs a differentially precoded binary DATA (P-DATA). Differential preceding (not to be confused with differential transmission line discussed below) is typically used to avoid error propagation at the receiver. The precoded binary P-DATA signal propagates along a differential transmission line 122 to the DA 150. The corresponding differential outputs of DA 150 include a first amplified precoded electronic binary signal, and a second precoded electronic binary signal; the second electronic binary signal being the logical inverse of the first electronic signal. Electronic signals and are typically binary non-return-to-zero (NRZ) AC-coupled data signals. Electronic signal is input into a first electronic duobinary filter 130, and electronic signal is input into a second identical electronic duobinary filter 140. The electronic duobinary filters 130 and 140 are typically based on either a delay-and-add filter (DAF) circuit with bit period delay or a low-pass filter (LPF) which approximates the action of DAF. Electronic duobinary filter 130 converts the 2-level binary signal B into a 3-level electronic duobinary signal, and electronic duobinary filter 140 similarly converts the 2-level binary signal into a 3-level electronic duobinary signal. The two complementary 3-level electronic duobinary signals and are applied to the two arms of the MZM, their amplitudes typically adjusted by DA to reach the proper voltage levels according to the required switching voltage of the MZM. Note that and carry the same duobinary data, but are complementary signals. Thus, thinking of duobinary as a 3-level format with symbols [+V, 0, −V], when swings high to +V, swings low to −V, however, when is 0, is also 0. A continuous wave light (CW) light source (LS) 160, such as a Distributed Feedback (DFB) semiconductor laser, outputs an optical carrier wave (OCW) having a wavelength λ. The duobinary MZM 170 modulates the OCW with the electronic duobinary signal, and outputs an optical duobinary signal O-DB.
An optical duobinary transmitter can also employ the single-drive MZM, shown in FIG. 1C. The single-drive MZM requires only a single electronic duobinary drive signal. However, a factor of 2 larger drive voltage is typically necessary in this case to drive the MZM between two transmission maxima to produce the optical duobinary modulation.
A schematic diagram for the conventional electronic DAF 100D, which can be used as the electronic duobinary generating filter with either a dual-drive or single-drive MZM, is shown in FIG. 1D. As shown, the DAF 100D includes an electronic splitter 182, a delay element 186, transmission lines 185, 187, 189, and a coupler or addition circuit 188. The propagation delay ΔT of transmission line 185 is nominally equal to the delay of transmission lines 187 and 189 combined. In conventional duobinary systems, the delay element 186 has a delay equal to a bit period T, where 1/T is the bit rate of the binary data signal B input to the electronic duobinary generating filter. Thus, delay element 186 outputs a delayed binary data signal B(ΔT+T) to the addition circuit 188, giving an additional delay of T to this signal.
In operation, an incoming binary data signal B, such as a precoded binary data signal, is split by the electronic splitter 182 into two paths along transmission lines 185 and 187. The signal on transmission line 185 propagates with a nominal delay ΔT to the addition circuit 188. The signal on transmission line 187 reaches the addition circuit 188 with a total cumulative delay ΔT+T, with the extra delay T obtained by transmission through delay element 186. Thus, the addition circuit 188 adds a binary data signal B(ΔT) with a delayed replica B(ΔT+T), where the delayed signal is delayed by a bit period T. In time domain, the DAF induces a correlation between adjacent bits. In frequency domain, the DAF may be viewed as a comb filter with a first null occurring at frequency ½ T. The process of passing a binary data signal B through DAF 100D produces a 3-level electronic duobinary signal having a spectrum compressed by a factor of 2 compared to the binary data signal B. For example, for a polar 2-level binary input data signal B with symbols [+V/2, −V/2], the duobinary filter 100D in FIG. 1D produces an 3-level signal with symbols [+V, 0, −V].
In fiber-optic communication systems, such as long-haul systems based on dense wavelength division multiplexing (DWDM), various deleterious effects of the channel can reduce the maximum transmission distance possible before the signals must be regenerated. In particular, deleterious channel effects include fiber chromatic dispersion, polarization mode dispersion, fiber nonlinearities, and distortion due to propagation through narrow optical DWDM filters, including possible distortions due to drift of a narrow optical filter off of a channel center frequency due to ambient temperature variation or other time varying environmental factors. The distortion due to narrow optical filters is particularly important in optical networks utilizing optical add/drop modules (OADM) that require DWDM de-multiplexing and multiplexing at each OADM node.
Yet, a flexible DWDM optical network requires the ability for individual wavelength channels to be added and/or dropped along the transmission line, which is typically accomplished with an OADM. Often, the architecture of an OADM requires that DWDM channels are first de-multiplexed, added/dropped, and then multiplexed again for transmission at each OADM node. This results in deleterious optical filtering imposed on the signals of each wavelength channel passed through the OADM, which can distort the signals, and cause performance degradation. Moreover, the amount of deleterious optical filtering imposed on a particular signal depends on the number of OADMs passed through by that signal. To mitigate these deleterious effects in optical DWDM networks typically requires an adaptable system. While optical duobinary systems are generally more tolerant to narrow optical filtering compared to other modulation formats due to their inherently narrow spectrum, eventually, transmission through multiple OADMs can also severely degrade the performance of a duobinary system, especially in high-spectral efficiency systems where the optical DWDM filter bandwidths approach the bitrate.
Different attempts have been made to generate improved optical duobinary signals that are more tolerant to various fiber-optic channel impairments. For example, to reduce the effect of chromatic dispersion in a fiber-optic communication system, Jae-Hoon Lee's patent application No. 2006/0072924 A1 (2006) teaches an optical transmitter based on converting an optical DPSK signal into a duobinary signal using an optical delay interferometer (DI), as shown in FIG. 2. According to Jae-Hoon Lee's scheme, a pre-coder 210 outputs a precoded binary signal 212 and a logically inverted precoded binary signal 214 into corresponding AMP 220 and AMP 230 respectively, to produce amplified signals 222 and 232. A laser light source 240 outputs a CW light 242, which DPSK MZM 250 modulates according to signals 222 and 232, to produce a modulated optical DPSK signal 252. Next, the modulated optical DPSK signal is input to an optical delay interferometer DI 300, having an optical splitter 260, an optical delay 270 and an optical coupler 280, to output an optical duobinary signal 282. The DI optical delay element has a delay in the range 0.5-0.8 of a bit period, with the delay set to improve chromatic dispersion tolerance. A similar scheme was proposed recently [Mikkelsen et. al., “Partial DPSK with Excellent Filter Tolerance and OSNR Sensitivity,” IEE Elect. Lett., Vol. 42, No. 23, 2006], whereby an optical DPSK signal is first propagated through the fiber-optic system, including narrow optical filters, and the delay of an optical DI demodulator is adjusted at the receiver to reduce the penalty from narrow optical filtering.
Both of the above schemes rely on optical filtering techniques, in particular utilizing an optical DI, to optimize the duobinary or DPSK signals. Indeed, Lee teaches against the conventional duobinary transmitter design based on an electronic duobinary generating filter, arguing that potential deleterious effects in the electronic driver amplifier may distort the duobinary signal (p. 1, paragraph 8). To avoid such distortions, Lee proposes eliminating the electronic duobinary generating filter and instead first generating an optical DPSK signal, and then passing the generated optical DPSK signals through an optical DI to produce the optical duobinary signal. In this way, Lee proposed to avoid the stated signal degradation caused by electronic amplifiers.
Lee's approach has the following shortcomings which need to be solved:                (1) Lee's design requires an optical DPSK MZM since it first generates an optical DPSK signal. As noted above, a DPSK MZM requires a greater bandwidth compared to an optical duobinary MZM, which can be a serious limitation in high-speed systems, for example.        (2) Lee's DI circuit 300 for generating the optical duobinary signal, being an optical mechanism, is difficult to adjust dynamically in real time to adapt for dynamically changing channel conditions. For example, to adjust the optical delay in a DI, typically some mechanical mechanism is used to vary the optical path length, which may be a relatively slow and bulky mechanism.        (3) Lee's optical DI circuit 300, being an optical mechanism with multiple components, is expensive and difficult to fabricate. The optical DI may also require a heater and/or thermo-electric cooler (TEC), and stabilization feedback system to accurately control the optical delay, as well as the optical phase. A frequency drift of only a few percent relative to bit rate can result in significant performance penalty [see for example, H. Kim and P. Winzer, “Robustness to Laser Frequency Offset in Direct-Detection DPSK and DQPSK Systems,” J. Lightwave Tech., Vol. 21, No. 9, p. 1887, September., 2003].        (4) Lee's optical DI circuit 300 is relatively large which makes it an unattractive optical element to be integrated into a Photonic Integrated Circuit (PIC), where space is limited and typically reserved for indispensable photonic elements. Integration of a DI circuit may unnecessarily substantially increase fabrication cost of a typical PIC transmitter.        
Thus, there is still a need for an improved optical duobinary transmitter which is adaptable to compensate for deleterious system impairments, such as in DWDM systems having different numbers of OADMs along the transmission line. This transmitter should be capable of high-speed operation, while also being reliable, low cost, and dynamically adaptable to deleterious channel dynamics, such as the drift of a narrow optical filter off of a channel center frequency due to ambient temperature variation. Moreover, such an adaptable transmitter should be attractive for integration on a PIC.
In one aspect, the present invention provides an adaptable optical duobinary transmitter which can compensate for certain deleterious system impairments while overcoming the shortcomings of the prior art. Despite Lee's teaching away from an electronic duobinary generating filter, the present invention is based on an electronic duobinary generating filter providing at least the following advantages over the prior art:                (1) The present invention provides an adaptation mechanism for optimizing the electrical and/or optical duobinary signal to mitigate certain system impairments.        (2) According to one aspect of the present invention, an adaptable electronic duobinary signal is generated by an adaptable electronic duobinary filter, which allows for a narrower bandwidth optical duobinary modulator, such as a duobinary MZM, in the transmitter. As this invention allows for an optical duobinary modulator which requires less bandwidth compared with an optical DPSK MZM used by some prior art, this invention improves performance, especially in high-speed systems.        (3) According to another aspect of this invention, an adaptable electronic duobinary generating filter is provided which is relatively easier to adjust dynamically in real time in order to adapt for dynamically changing channel impairments.        (4) According to yet another aspect of this invention, the adaptable electronic generating filter is relatively inexpensive and easier to fabricate than an optical duobinary generating filter, such as the optical DI suggested by the prior art.        (5) According to an additional aspect of the present invention, an adaptable electronic mechanism is provided which can be more easily miniaturized compared with a multi component optical mechanism, such as the optical DI provided by Lee.        (6) According to yet another aspect of the present invention, an adaptable optical duobinary transmitter is provided which is attractive for photonic integration on a PIC.        
These and other advantages related to this invention will become apparent and appreciated by referring to the following summary, description, and claims taken in conjunction with the accompanying drawings.